The invention is directed to a process for the production of ceramic fuel pellets which contain the fission and/or breeder (fertile) material in oxidized form by heating corresponding green premolded pellets to a temperature below 2500.degree. C., quickly transferring them into a steel die, impact compressing at a molding pressure below 1000 MN/m.sup.2 within a few milliseconds, ejecting the pellets and cooling.
The fuel elements for light water (LWR) and fast breeden sodium cooled reactors (LMFBR) contain the fuel and/or fertile material predominantly in the form of pellets made of uranium oxide or a mixture of uranium oxide and plutonium oxide. The fission material plutonium is preponderantly used in SNR reactors; however, it is also employed in limited amounts in the LWR reactors in place of U.sup.235.
The fuel pellets are customarily produced from uranium oxide powder (UO.sub.2) or a mixture of powdered oxides UO.sub.2 and PuO.sub.2 by molding, sintering, and grinding. These molding-sintering processes have a number of disadvantages. The geometric density of the fuel tablets after the molding is small and limited by the maximum permissible pressure of 500 to 1000 MN/m.sup.2 of the work piece to less than 70% of the theoretical density. Consequently, an expensive post densification by sintering is necessary. In order to guarantee the quality of the sintered pellets reproducibly, there were placed high requirements on the molding powder in regard to the purity and sinterability, which only could be fulfilled by expensive conversion processes in the production of UO.sub.2 or UO.sub.2 /PuO.sub.2 molding powders (e.g., German Pat. No. 1592468, German AS No. 1592477, or German OS No. 2623977 and related Borner U.S. Pat. No. 4,152,395). As a result, the presses used and the sintering furnace which is about 13 meters long require a lot of space.
Especially in the processing of plutonium containing fuel the boxing in of large apparatuses is expensive and associated with high costs of maintenance, supervision, and operation.
The diameter tolerances of the fuel pellets are .+-.25 .mu.m. In order to maintain this, all pellets after the sintering are circular to the required dimensions. The loss of weight in the grinding is 2 to 4%. Besides, there cannot be avoided waste of about 10% through breakage of edges and through spalling.
The processing above all of the plutonium containing grits has been found to be most difficult and is associated with a very high expense.
The sintered fuel pellets in the reactor are frequently inclined to post sintering. The post densification connected therewith contributes to enlarging the heat checking split between the pellets and the fuel tubes and therewith the heat transfer is deteriorated. Furthermore, the reactivity in the reactor nucleus as a result of increased temperature is unfavorable.
Besides the preparation of fuel pellets, according to the molding-sintering process, there have been attempts to produce the fuel pellets by hot presses. At the required temperatures of more than 1300.degree. C., however, the work material problems for the mold and die could not be solved economically. The changes in dimension tolerances producible thereby nevertheless require an expensive post processing. Therefore, this process has not acquired any industrial significance.
In order to solve the work material problems created by the presses at high temperature and pressure, there has been proposed the manufacture of the fuel pellets according to a hot impact compression process (HSV process) (German Pat. No. 2842402 and related Hrovat U.S. Pat. No. 4,271,102). In the HSV process, an green pellet capable of being handled is heated to a temperature above 1300.degree. C. and compressed and shaped in a cold die so fast that there cannot occur any mentionable heat exchange between the pellet and tool. An essential feature of the process is that heat and pressure, in contrast to other hot pressing processes, are substantially deconpled, effecting the pellet jointly for a very short time (about one millisecond).
Through this, all interactions between pellet and tool which require a certain period of time are avoided. The pellet leaves the die while it is still in the plastic temperature region; however, it must have the required form stability in order to withstand without damage the dynamic stresses of the ejection process. The compressive stress as a result of the build up pressure by heating up in the densification of the porous pellets proves to be of the utmost importance in this process. In order to avoid it, it is necessary to carry out the heating densification and ejection of the pellets in a vacuum. According to this process, carbidic fuel tablets (UC and UC/PuC) are produced without problem.
In contrast, in the production of oxide fuel pellets, there occur a number of difficulties. The two oxides UO.sub.2 and PuO.sub.2 under the same conditions compared to uranium and plutonium carbide have a higher vapor pressure by a factor of about 500. Therefore, there occurs in the vacuum such a high rate of vaporization that it is not possible to use the HSV process industrially without modification. Besides, since the vapor pressure of PuO.sub.2 is substantially higher than that of UO.sub.2, there can occur at relatively high vaporization mate a selective enrichment of the PuO.sub.2 component in the vapor phase, and therewith a segregation of uranium and plutonium in the pellets. The vaporized oxide condenses on the cold places in the heating oven and prevents the conveyance of the pellets.
Furthermore, the relatively high thermoshock sensitivity of oxidic fuel in the heating and cooling steps leads to numerous shrinkage cracks in the inside of the pellets.
The invention, therefore, is based on the problem of developing a process for the production of ceramic fuel pellets which contain the fission and/or fertile material in oxidic form, by heating corresponding crude pellets to a temperature below 2500.degree. C., quickly transferring them into a steel die, impact compressing at a pressing pressure below 1000 MN/m.sup.2 within a few milliseconds, ejecting the pellets and cooling in which no segregation and deposition of vaporized oxide occurs in the furnace, and above all no shrinkage cracks in the inside of the tablets.